Waveguide Display Device

ABSTRACT

An optical waveguide includes an input diffractive optical element arranged for being aligned with an optical projector for diffracting the light beam therefrom, a waveguide substrate arranged for reflecting the light beam diffracted by the input diffractive optical element by means of total internal reflection, and an output diffractive optical element coupled at said waveguide substrate for partially diffracting the light beam as a diffracted light and partially transmitting the light beam as a transmitted light during the total internal reflection of the light beam within the waveguide substrate. The diffracted light is diffracted by the output diffractive optical element and is projected out of the waveguide substrate toward the user eye. The transmitted light is continuously transmitted and reflected within the waveguide substrate by the total internal reflection until the transmitted light is totally diffracted out of the waveguide substrate, so as to complete an exit pupil expansion.

CROSS REFERENCE OF RELATED APPLICATION

This is a Divisional application that claims the benefit of priorityunder 35U.S.C.§ 120 to a non-provisional application, application Ser.No. 17/152,737, filed Jan. 19, 2021, which is a Divisional applicationthat claims the benefit of priority under 35U.S.C.§ 120 to anon-provisional application, application Ser. No. 16/695,141, filed Nov.25, 2019, now U.S. Pat. No. 10,962,787, which are incorporated herewithby references in their entirety.

BACKGROUND OF THE PRESENT INVENTION Field of Invention

The present invention relates to an augmented reality display device(AR) or a head-up display device, and more particularly to a waveguidedisplay device, which is able to increase a brightness uniformity of apupil output image and to improve a system output efficiency.

Description of Related Arts

Augmented reality is a new technology to provide an interactiveexperience of a real world environment where virtual objects are residedin the real world in a real time manner. Accordingly, acomputer-generated prompt message, virtual object, or virtual scene issuperimposed into the real world to achieve the interactive experiencefor the users. Applications of augmented reality can be widely used indata model visualization, military weapon development and manufacturing,flight navigation, medical training, remote control, entertainment andart, in order to enhance display output in real-world environments.

There are two major types of display devices in augmented realitytechnology, which are an optical transmission type and a videotransmission type. Optical transmission augmented reality displaysystems have become mainstream due to their high resolution, no visualbias, no delay, and better social habits.

In order to configure an optical transmission augmented reality display,a conventional optical system based on a Bird Bath or a free-formsurface element has been designed for forming the superposition ofvirtual and real worlds with refraction and reflection. However, sincethe display system is constructed by conventional optical components, itis limited by the overall optical distance to minimize a thinness of thedisplay system. The wear-ability of the display system as a spectaclecannot be achieved. Furthermore, due to the constraints of theLagrangian invariant, the size of the conventional optical displaysystem is limited, and the optical display system cannot perfectly matchwith the pupils of the user. The waveguide display device caneffectively solve the above two problems comparing to the conventionaloptical systems. After a single color or RGB image is inserted into thewaveguide, the total internal reflection of the light in the planarwaveguide element is utilized, to effectively reduce the thickness ofthe optical component. By using one or more optical components on thewaveguide to control the image sequencing output, an exit pupilexpansion is achieved. Based on the exit pupil expansion in thewaveguide display device, the light energy in the waveguide is graduallyattenuated during the image sequencing output process, and thediffraction efficiency of the input-and-output optical components on thediffraction waveguide is low, such that the system energy issignificantly lost. Therefore, how to achieve a uniform image brightnessand improve system transmission efficiency are major concerns to improvethe user experience and system performance of augmented reality displaysor heads-up displays.

Accordingly, Chinese patent, CN104280885A entitled “large exit pupilholographic waveguide glass system” disclosed a method of interferinglight intensity conversion phase, wherein it can only be used forrefractive index modulated holographic grating waveguides. Chinesepatent, CN107690599A entitled “Optical Display System”, and US patent,U.S. Pat. No. 9,329,325 entitled “Optical Waveguides” disclose a methodof plating one or more layers of gradient film on a monolithic opticalwaveguide to change the diffraction efficiency. However, the gradientcoating process is complicated and the manufacturing cost is relativelyhigh since the process is performed on a single optical waveguide.

SUMMARY OF THE PRESENT INVENTION

The invention is advantageous in that it provides a waveguide displaydevice, which is able to increase a brightness uniformity of a pupiloutput image, to improve a system output efficiency and to reduce themanufacturing cost.

Another advantage of the invention is to a waveguide display device,wherein the grating structure is modulated to provide a uniformbrightness of the image being seen by the user, even though there is anyrelative displacement of the AR spectacle with respect to the user eyes,the brightness of the image will remain uniform and stable (thebrightness in the window eyebox is uniform).

Additional advantages and features of the invention will become apparentfrom the description which follows, and may be realized by means of theinstrumentalities and combinations particular point out in the appendedclaims.

According to the present invention, the foregoing and other objects andadvantages are attained by a waveguide display device comprising awaveguide substrate, an input diffractive optical element and an outputdiffractive optical element.

The input diffractive optical element is arranged for coupling an outputlight of a light projector to the waveguide substrate.

The waveguide substrate is arranged for reflecting light from the inputdiffractive optical element by means of total internal reflection towardthe output diffractive optical element.

The output diffractive optical element for partially diffracting andpartially transmitting light reflected by the waveguide substrate viathe total internal reflection, wherein at each light reflection, thediffracted light is projected out of the waveguide substrate toward auser eye, and the transmitted light is continuously reflected within thewaveguide substrate by means of total internal reflection until thelight is diffracted out of the waveguide substrate for completing anexit pupil expansion.

The input diffractive optical element is optimized to have high couplingdiffraction efficiency for improving display system efficiency andreducing power consumption. The input diffractive optical element isselected from a flitting grating, an asymmetric surface relief grating,or other diffractive structure having high coupling efficiency.

The waveguide substrate can be a flat plate structure made of hightransparent optical material for visible light passing through, whereinthe upper and lower surfaces thereof are extended parallel. The inputdiffractive optical element and the output diffractive optical elementcan be closely adhered to the surface of the waveguide substrate, or canbe embedded in the waveguide substrate.

The output diffractive optical element is incorporated with a periodicstructure with low coupling diffraction efficiency to ensure continuouslight energy output during the process of the exit pupil expansion. Thecoupling diffraction efficiency is modulated in an effective region ofthe output diffractive optical element, wherein the modulation methodcan be a regional modulation or continuous modulation. The regionalmodulation or continuous modulation is optimized according to the pathof the exit pupil expansion.

For maximizing the efficiency of the waveguide display system, byimproving the coupling diffraction efficiency of the input diffractiveoptical element, the output diffractive optical element is arranged tooutput all of the light energy in the waveguide substrate in itseffective region when the light is totally reflected therewithin. At thesame time, in order to provide a uniform brightness of an image withinthe range of the exit pupil of the waveguide display device, the outputdiffractive optical element is arranged to maintain a constant lightflux output per unit area in its effective area. According to thecharacteristics of exit pupil expansion with sequencing energy output bythe output diffractive optical element, the coupled output diffractionefficiency of the output diffractive optical element is optimized to:

$\begin{matrix}{\eta_{N} = \frac{1}{N_{T} - \left( {N - 1} \right)}} & (1)\end{matrix}$

wherein N is the output order of the regional modulation, N_(T) is thetotal number of modulation regions, and η_(N) is the coupling efficiencyof the Nth region.

Due to the limitations of the structure or machine accuracy of thediffractive optical element, when the coupling output efficiency of theoutput diffractive optical element after optimization cannot be equal tothe result given by the equation (1), a portion of the light energy isgenerated by the total internal reflection in the waveguide substratewithin its effective coupling area. Assuming that the ratio of thisportion of the light energy to the total light energy of total internalreflection is ητ, in order to provide the uniform brightness of theimage within the range of the exit pupil, the coupling output efficiencyof the output diffractive optical element is optimized to:

$\begin{matrix}{\eta_{N} = \frac{\eta_{T}}{N_{T} - \left( {N - 1} \right)}} & (2)\end{matrix}$

The output diffractive optical element can be selected as aone-dimensional diffraction grating, wherein the diffraction efficiencyis modulated based on the parameters of the diffraction structure. Inother words, by changing the position of the waveguide surface, theone-dimensional diffraction grating structure can be optimized, toensure the coupling output diffraction efficiency of the outputdiffractive optical element following the equation (1) or (2). Forexample, a one-dimensional straight tooth surface relief grating canmodulate the duty cycle and the tooth height, a one dimensional slantedtooth surface relief grating can modulate duty cycle, tooth height andtilt angle, and a one-dimensional blazed grating can modulate toothheight and surface angle.

The output diffractive optical element can be selected as atwo-dimensional periodic diffractive structure, wherein the diffractionefficiency is modulated based on the parameters of the diffractionstructure. In other words, by changing the position of the waveguidesurface, the two-dimensional periodic diffraction grating structure canbe optimized, to optimize the coupling output diffraction efficiency ofthe output diffractive optical element. For example, a two-dimensionalcolumnar diffractive structure can modulate the duty cycle and the toothheight.

The etching process of the present invention can be one of electron beametching, reactive ion beam etching, magnetically enhanced reactive ionetching, high density plasma etching, inductively coupled plasmaetching, pressure swing coupled plasma etching, and electrons cyclotronresonance etching. The modulation tooth height can be achieved bycontrolling the etching time and exposure intensity of the electron beamor ion beam on the master substrate. For mass production, the gratingstructure on the master substrate can be copied into the replica resinmaterial by nano imprinting, casting, molding, injection molding, andthe like in order to reduce the manufacturing cost.

In accordance with another aspect of the invention, the presentinvention comprises

Still further objects and advantages will become apparent from aconsideration of the ensuing description and drawings.

These and other objectives, features, and advantages of the presentinvention will become apparent from the following detailed description,the accompanying drawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a spectacle frame of a waveguide displaydevice with an optical transmission augmented reality system accordingto a preferred embodiment of the present invention.

FIG. 2 is a side view of the spectacle frame of the waveguide displaydevice with the optical transmission augmented reality system accordingto the above preferred embodiment of the present invention.

FIG. 3 is a sectional view of the waveguide display device with theoptical transmission augmented reality system according to the abovepreferred embodiment of the present invention.

FIG. 4 is a top view of the waveguide display device with the opticaltransmission augmented reality system according to the above preferredembodiment of the present invention.

FIG. 5 illustrates a one-dimensional linear diffraction two dimensionalexit pupil optical waveguide according to the above preferred embodimentof the present invention.

FIG. 6A illustrates a one dimensional straight tooth surface reliefgrating according to the above preferred embodiment of the presentinvention.

FIG. 6B illustrates a one dimensional slanted tooth surface reliefgrating according to the above preferred embodiment of the presentinvention.

FIG. 6C illustrates a one dimensional blazed grating according to theabove preferred embodiment of the present invention.

FIG. 7A illustrates the one dimensional straight tooth surface reliefgrating with depth modulation based on the bottom of the grating toothaccording to the above preferred embodiment of the present invention.

FIG. 7B illustrates the one dimensional straight tooth surface reliefgrating with depth modulation based on the top of the grating toothaccording to the above preferred embodiment of the present invention.

FIG. 8A is a table illustrating a comparison of an output diffractionefficiency of the regional depth modulated slanted tooth surface reliefgrating and the non-modulated slanted tooth surface relief grating.

FIG. 8B is a table illustrating a comparison of an output light flux ofthe regional depth modulated slanted tooth surface relief grating andthe non-modulated slanted tooth surface relief grating.

FIG. 9A is a front view of a two dimensional linear diffraction twodimensional exit pupil waveguide according to the above preferredembodiment of the present invention.

FIG. 9B is a front view of a two dimensional cylindrical diffraction twodimensional exit pupil waveguide according to the above preferredembodiment of the present invention.

FIG. 10A is a schematic view of a two dimensional pyramid diffractionstructure according to the above preferred embodiment of the presentinvention.

FIG. 10B is a schematic view of a two dimensional checkerboarddiffraction structure according to the above preferred embodiment of thepresent invention.

FIG. 10C is a schematic view of a two dimensional columnar diffractionstructure according to the above preferred embodiment of the presentinvention.

FIG. 10D is a schematic view of a two dimensional linear obliquediffraction structure according to the above preferred embodiment of thepresent invention.

FIG. 10E is a schematic view of a two dimensional linear orthogonaldiffraction structure according to the above preferred embodiment of thepresent invention.

FIG. 11A is a perspective view illustrating a two dimensional columnardiffraction structure and a related Cartesian coordinate system, whereinthe X-axis of the coordinate system is parallel to a certain periodicdirection.

FIG. 11B is a top view illustrating a two dimensional columnardiffraction structure and its related structural parameters according tothe above preferred embodiment of the present invention.

FIG. 12 is a polar coordinate system defining a light incident directionaccording to the above preferred embodiment of the present invention.

FIG. 13A is a table illustrating a comparison of the modulated dutycycle and non-modulated duty cycle for the two-dimensional columnardiffraction structure (1,1) order transmission diffraction efficiency.

FIG. 13B is a table illustrating a comparison of the modulated dutycycle and non-modulated duty cycle for the two-dimensional columnardiffraction structure (1,1) level output flux.

FIG. 14 is a table showing the relevant parameters of the system and thegrating.

FIG. 15 is a table showing the tooth profile depth of the grating ineach modulation region.

FIG. 16 is a table showing the relevant parameters of the system, thediffraction structure, and the angle of incidence.

FIG. 17 is a table showing the duty cycle of the columnar structure ineach modulation region.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following description is disclosed to enable any person skilled inthe art to make and use the present invention. Preferred embodiments areprovided in the following description only as examples and modificationswill be apparent to those skilled in the art. The general principlesdefined in the following description would be applied to otherembodiments, alternatives, modifications, equivalents, and applicationswithout departing from the spirit and scope of the present invention.

Referring to FIG. 1 of the drawings, an augmented reality device forbeing worn by a user is illustrated, wherein the augmented realitywearable device comprises a spectacle frame 4 and a waveguide displaydevice. The spectacle frame 4 comprises two spectacle arms and a bridge,and an augmented reality system 1, which is based on an opticalwaveguide. The augmented reality system 1 comprises a waveguide displaydevice which comprises two optical waveguides, i.e. a left-eye opticalwaveguide 2 and a right-eye optical waveguide 3, wherein the bridge isextended between two inner sides of the optical waveguides and thespectacle arms are extended from two outer sides of the opticalwaveguides respectively. The augmented reality system 1 furthercomprises a computing module 5, a positioning sensor 6, an externalspace collector 7, and a remote computing system 8. The left-eye opticalwaveguide 2 and the right-eye optical waveguide 3 have a high lighttransmittance for the user to clearly see through the left-eye opticalwaveguide 2 and the right-eye optical waveguide 3 to view the realworld. The computing module 5 is provided at, preferably built-in with,a spectacle arm for generating corresponding image signals for the leftand right eyes of the user when wearing the spectacle frame 4 in orderto provide a three-dimensional stereoscopic experience for the user. Thecomputing module 5 is further operatively connected with varioussensors, such as the positioning sensor 6 and the external spacecollector 7, in the system at the same time. The positioning sensor 6 isarranged to determine the position and orientation in a given coordinatesystem for the augmented reality system, wherein the positioning sensor6 includes the degree of freedom of movement in the direction of thethree orthogonal coordinate axes and the degree of freedom of rotationabout the three coordinate axes. Accordingly, the positioning sensor 6can be a combination of an accelerometer, a gyroscope, a magnetometer,and a global positioning system receiver. After the computing module 5is operated to process an output of the position sensor 6, the virtualobject is accurately rendered in the real world. The external spacecollector 7 is built-in at the bridge and can be a combination of a RGBcamera, a monochrome camera, and a depth camera. An RGB camera or amonochrome camera is arranged to acquire a real scene of the externalenvironment, wherein the depth camera is arranged to acquire depthinformation of the external environment. When the optical axes of thedepth camera and other cameras are parallel and synchronized in a timemanner, a complete information of the actual scene can be obtained. Theremote computing system 8 is arranged to provide an additional computingpower to the computing module 5 at the spectacle frame 4 by connectingthe computing module 5 with the remote computing system 8 by wire orwireless connection. Alternatively, the remote computing system 8 can bethe only computing means to replace the computing module 5 for datacomputation in the augmented reality system 1.

As shown in FIG. 2 , the augmented reality system 1 further comprises anoptical projector 9 built-in with at least one of the spectacle arms ofthe spectacle frame 4 to form a compact structure. The optical projector9 comprises a micro-display 11 for projecting an image in form of thelight beam and a lens set 10, wherein after the image is amplified bythe lens set 10, the image is input to the left-eye optical waveguide 2through an input diffractive optical element 12 coupled thereto.Accordingly, the micro-display 11 can be a liquid crystal display (LCD)with high light transmittance for forming the image via a projectionmodulation of liquid crystal molecules on the backlight. Alternatively,the image can be formed by reflective modulation method, such as digitallight processors (DLPs) and liquid crystal on silicon (LCoS). It isworth mentioning that the micro-display 11 can also incorporate with aself-illuminating organic light emitting diode (OLED) and a micro lightemitting diode (Micro LED). The micro-display 11 can also incorporatewith a MEMS Scanning Mirror. The lens group 10 is constructed with oneor more optical lenses for image amplification.

As shown in FIG. 3 , the right-eye optical waveguide 3 is coupled at aright side of the spectacle frame 4, wherein the right-eye opticalwaveguide 3 comprises the input diffractive optical element 12 and anoutput diffractive optical element 13. Accordingly, after the lightdiffracted by the input diffractive optical element 12 is guided toinput in the right-eye optical waveguide 3, the light is totallyreflected and transmit to the output diffractive optical element 13 soas to project to the user eye.

As shown in FIG. 4 , the light beam 15 generated by the pixel of themicro-display 11 is collimated by the lens set 10 in the opticalprojector 9. After being diffracted by the input diffractive opticalelement 12, the first diffraction order light beam 16 satisfies thetotal internal reflection condition of the waveguide. Therefore, thefirst diffraction order light beam 16 is arranged to be fully reflectedin a waveguide substrate 14 of the right-eye optical waveguide 3 towardthe output diffractive optical element 13. At each time the firstdiffraction order light beam 16 contacts with the output diffractiveoptical element 13, the first diffraction order light beam 16 ispartially transmitted and is partially diffracted. The diffracted light17 is guided to projected out of the waveguide substrate 14 toward theuser eye. The transmitted light is guided to continuously transmit inthe waveguide substrate 14 in total internal reflection until the firstdiffraction order light beam 16 is guided to diffract of the waveguidesubstrate 14 toward the user eye, so as to complete the exit pupilexpansion along a Y-direction.

It is worth mentioning that the waveguide substrate 14 has a diffractiveside and an opposed projecting side, wherein the input diffractiveoptical element 12 and the output diffractive optical element 13 arelocated at the diffractive side of the waveguide substrate 14 forguiding the diffracted lights 17 being projected out the waveguidesubstrate 14 through the projecting side thereof.

Accordingly, the present invention is able to apply to differentconfigurations of the diffraction waveguide augmented reality. Theaugmented reality device based on a one-dimensional linear diffractionwaveguide will be described as the following example. FIG. 5 illustratesa one-dimensional linear diffraction waveguide, which comprises an inputdiffractive optical element 12, an output diffractive optical element13, and a transmission diffractive optical element 18. The configurationis able to perform a two-dimensional extension. The input image of theoptical projector (not shown) is in the direction of propagation of thefield of view centering on the plane orthogonal to the plane of theone-dimensional linear diffraction waveguide 19. The input diffractiveoptical element 12 is arranged to diffract the input light, wherein thefirst diffraction order satisfies the total internal reflectioncondition of the waveguide substrate, such that the light is guidedtoward the transmission diffractive optical element 18 in the substrate.The total reflected light in the one-dimensional linear diffractionwaveguide 19 will be transmitted or diffracted each time when the lightcontacts with the transmission diffractive optical element 18. Thetransmission direction of the first diffraction order generated by thediffraction of the transmission diffractive optical element 18 isdeflected to satisfy the total internal reflection condition of thewaveguide substrate, such that the light is guided toward the outputdiffractive optical element 13 in the substrate. The light directlytransmitted through the transmission diffractive optical element 18 willbe continuously reflected via the total internal reflection until thelight is diffracted and transmitted out of the output diffractiveoptical element 13 so as to complete the one-dimensional exit pupilexpansion along a X-direction. Furthermore, the light from thetransmission region is also partially diffracted and partiallytransmitted each time when the light contacts the output diffractiveoptical element 13. The diffracted light is projected out of theone-dimensional linear diffraction waveguide 19 toward the user eye. Thetransmitted light is guided to be continuously reflected via the totalinternal reflection in the one-dimensional linear diffraction waveguide19 until the light is diffracted and transmitted out of the outputdiffractive optical element 13 so as to complete the one-dimensionalexit pupil expansion along a Y-direction. The lights are successivelyexpanded in the X and Y dimensions, such that the waveguide displaydevice as shown in FIG. 5 can perform the two-dimensional exit pupilexpansion by diffracting the light through the diffractive opticalelement 18 and outputting the light from the output diffractive opticalelement 13. The waveguide display device can be configured as shown inFIG. 5 , to include only the input diffractive optical element and theoutput diffractive optical element, but the transmission diffractiveoptical element is omitted. In one example, after the input light isdiffracted by the input diffractive optical element, the light is guidedto directly reflect toward the output diffractive optical element fortotal internal reflection, such that the light is guided to graduallydiffract by the output diffractive optical element so as to complete theone-dimensional expansion.

FIGS. 6A to 6C illustrate three different configurations of theone-dimensional linear diffraction grating. The input diffractiveoptical element 12, the output diffractive optical element 13 and thetransmission diffractive optical element 18 can all be selectivelyconstructed in the above different configurations. FIG. 6A illustrates aone dimensional straight tooth surface relief grating 20 which comprisesa toothed unit perpendicularly extended to the waveguide substrate,wherein its associated tooth profile parameters can be a grating periodΛ, a tooth width w and a tooth height h. FIG. 6B illustrates a onedimensional slanted tooth surface relief grating 21, wherein the toothedunit is slantedly extended to the waveguide substrate with an angle α,and its associated tooth profile parameters can be a grating period Λ, atooth width w and a tooth height h. FIG. 6C illustrates a onedimensional blazed grating 22, wherein an angle between the blazedsurface and the non-blazed surface is β, the grating period is Λ, andthe tooth height is h. Accordingly, each of the grating structures shownin FIG. 6A-6C has a constant tooth profile. In other words, theparameters associated with the tooth profile do not change with respectto the position of the grating at the waveguide face, i.e., the gratingstructure is non-modulated. However, the diffraction efficiency of thegrating is determined by parameters of the tooth shape, wherein thenon-modulated grating structure has the same diffraction efficiency forthe incident light in the same direction. Therefore, since the outputdiffractive optical element is needed to gradually output the lightenergy within its effective region in the waveguide substrate via thetotal internal reflection, the same diffraction efficiency will causethe reduction of luminous flux output at each step output. Therefore, itis impossible to obtain a uniform brightness image to the user withinthe exit pupil range of the waveguide.

In order to solve the above problems, the present invention provides amodulation grating structure. In other words, via the method of changingthe tooth shape with the position of the grating on the waveguidesurface, the diffraction efficiency of coupling output of the outputdiffractive optical element at different positions will be different.According to the characteristics of the energy sequencing output of theoutput diffractive optical element for the exit pupil expansion, whenthe coupling output diffraction efficiency of the output diffractiveoptical element is optimized to form the result given by the equation(1), the luminous flux output per unit area in the effective area iskept constant, such that the brightness of the image projected to theuser is uniform and the efficiency of the waveguide display device ismaximized. In other words, the image brightness is maximized under acertain system power consumption or the system power consumption isminimized at a certain image brightness.

$\begin{matrix}{\eta_{N} = \frac{1}{N_{T} - \left( {N - 1} \right)}} & (1)\end{matrix}$

wherein N is the output order of the regional modulation, N_(T) is thetotal number of modulation regions, and η_(N) is the coupling efficiencyof the Nth region.

It is worth mentioning that the equation (1) is used for providing auniform brightness of the image visible to the user, and to ensure thelight flux output per unit area in the AR output diffraction gratingregion to be constant.

Due to the limitations of the structure or machine accuracy of thediffractive optical element, when the coupling output efficiency of theoutput diffractive optical element after optimization cannot be equal tothe result given by the equation (1), a portion of the light energy isgenerated by the total internal reflection in the waveguide substratewithin its effective coupling area. Assuming that the ratio of thisportion of the light energy to the total light energy of total internalreflection is Tyr, in order to provide the uniform brightness of theimage within the range of the exit pupil, the coupling output efficiencyof the output diffractive optical element is optimized to the equation(2).

$\begin{matrix}{\eta_{N} = \frac{\eta_{T}}{N_{T} - \left( {N - 1} \right)}} & (2)\end{matrix}$

In order to optimize the coupling output efficiency, sensitiveparameters having a significant influence with respect to thediffraction efficiency should be selected. The diffraction efficiency ofthe straight tooth surface relief grating 20 is arranged for beingchanged by modulating the duty cycle ff=w/A or the tooth height h. Theslanted tooth surface relief grating 21 can modulate the duty cycle, thetooth height or the tilt angle α. The blazed grating 22 can modulate thetooth height or the face angle β.

Taking the tooth height of the embossed grating on the surface of theslanted tooth as an example, the tooth height is arranged to be changedaccording to the position of the grating on the waveguide surface.Depending on the manufacturing method, the specific structure can bemodulated by the method based on the bottom of the grating tooth asshown in FIG. 7A or the method based on the top of the grating tooth asshown in FIG. 7A. The etching process of the present invention can beone of electron beam etching, reactive ion beam etching, magneticallyenhanced reactive ion etching, high density plasma etching, inductivelycoupled plasma etching, pressure swing coupled plasma etching, andelectrons cyclotron resonance etching. The modulation tooth height canbe achieved by controlling the etching time and exposure intensity ofthe electron beam or ion beam on the master substrate. For massproduction, the grating structure on the master substrate can be copiedinto the replica resin material by nano imprinting, casting, molding,injection molding, and the like. The master substrate can be made byelectronic beam lithography and reactive ion beam etching.

In one example, assume that a refractive index of n=1.7, the modulatedslanted tooth surface relief grating has 12 depth modulated regions.FIG. 14 illustrates the relevant parameters of the system and thegrating. FIG. 15 illustrates the tooth profile depth of the grating ineach modulation region. As shown in FIG. 8A, the diffraction efficiencyof the depth-modulated slanted tooth surface relief gratingsignificantly increases as the grating depth increases. As shown in FIG.8B, the output light flux of each area is constant, such that the useris able to see the image with an uniform brightness. By comparing theconstant depth of tooth, the output total light flux of the modulatedslanted tooth surface relief grating is the same as that of thenon-modulated slanted tooth surface relief grating. As comparing theFIGS. 8A and 8B, the non-modulated slanted tooth surface relief gratinghas the constant output diffraction efficiency. Due to the distributionof the output energy of the light energy in the effective region of thewaveguide, the output light flux will rapidly decrease according to thepropagation distance within the range of the exit pupil. Therefore, thebrightness of the output image is uneven within the range of the exitpupil to affect the user's visual experience.

The manufacturing method of the optical waveguide comprises thefollowing steps.

(1) Form the master substrate by means of electron beam lithography.

(2) Etch the master substrate by using reactive ion beam process.

(3) Nano-imprint the master substrate for mass production andreplication by the following steps.

(a) Place a chemical precursor of an elastic sub-mold in the mastersubstrate, and after polymerization, remove the chemical precursor fromthe master substrate to form the elastic sub-mold, wherein the elasticsub-mold has a mold cavity formed with respect to the master substrate.

(b) Evenly apply a resin on a wafer substrate, wherein the resin can beUV solidifying resin or thermosetting resin.

(c) Mold the elastic sub-mold into the wafer substrate, preferably byapplying pressure, in order to fully fill the resin into the mold cavityof the elastic sub-mold, and perform polymerization and solidificationof the resin to form a polymer in an embossed area of the elasticsub-mold by ultraviolet light or heat.

(d) Remove the elastic sub-mold from the wafer substrate, such that thereplicated structure is formed on the wafer substrate.

The present invention can also be applied to the optical waveguide basedon a two-dimensional periodic diffraction structure. In another example,the operating principle of the optical waveguide is arranged for theaugmented reality device based on a two-dimensional linear diffractionwaveguide and a two-dimensional columnar structure diffractionwaveguide. FIGS. 9A and 9B illustrate the optical path of thetwo-dimensional linear diffraction waveguide 23 and the two-dimensionalcolumnar structure diffraction waveguide 26 respectively. The inputregion 24 of the two-dimensional linear diffraction waveguide 23 and theinput region 27 of the two-dimensional columnar diffraction waveguide 26can be configured as the one-dimensional linear diffraction structure ora two-dimensional periodic diffraction structure. The output region 25of the two-dimensional linear diffraction waveguide 23 and the outputregion 28 of the two-dimensional columnar structure diffractionwaveguide 26 can be configured as the two-dimensional periodicdiffraction structure. The periodicity of the two-dimensional periodicdiffractive structure has at least two directions, wherein two or moredirections of the periodicity can be perpendicular to each other or canbe formed with any angle. It is different from the one-dimensionaldiffraction waveguide which requires the transmission diffractiveoptical element and the output diffractive optical element to completethe two-dimensional exit pupil expansion. Due to the multiple directionsof the periodicity, the two-dimensional periodic diffractive structureallows the light being reflected by total internal reflection in thewaveguide substrate within the output region 25 of the two-dimensionallinear diffraction waveguide 23 or the output region 28 of thetwo-dimensional columnar diffraction waveguide 26 in order to completethe two-dimensional exit pupil expansion, so as to enlarge the viewingsize of the screen.

FIGS. 10A to 10E illustrate five different two-dimensional periodicdiffraction structures which are pyramid, checkerboard, columnar, linearoblique intersection, and linear orthogonal, but it should not belimited in the present invention. Similar to the above one-dimensionallinear diffraction grating, if the dimensional parameters of thetwo-dimensional periodic diffraction structure remain unchanged, thestructure is non-modulated and its diffraction efficiency remainsconstant. During the sequencing output of the two-dimensional expansionof the light in the two-dimensional diffraction output region, theoutput light flux at each sequencing output is decreased. Therefore, thebrightness of the image to the user is not uniform within the range ofthe exit pupil of the waveguide. When the present invention is appliedto the optical waveguide based on the two-dimensional periodicdiffraction structure, the relevant structural parameters of themodulated two-dimensional periodic diffraction structure can be used,such that the coupling output diffraction efficiency is successivelychanged during the two-dimensional expansion process so as to achievethe brightness uniformity of the output image. Alternatively, thestructural parameters can be modulated to change the diffractionefficiency.

FIGS. 11A and 11B illustrate the two-dimensional columnar diffractionstructure and its related parameters as an example. The X-axis of thecoordinate system is parallel to the direction of the first period, andthe Z-axis is perpendicular to the waveguide surface. The relevantstructural parameters are height h, column diameter d, first period Λ1and second period Λ2. FIG. 12 illustrates the light incident directiondefined by a polar angle Θ and an azimuth furnace in the coordinatesystem. Due to the periodicity in both two directions, the diffractionorder of the two-dimensional structure is expressed in the form of (m,n). The diffraction efficiency of the two-dimensional columnardiffractive structure can be changed by modulating the duty cycleffi=d/Λ{circumflex over ( )}ff2=d/Λ2 or the tooth height h. In oneexample of the modulation duty cycle, the columnar diffraction structurehas a refractive index η=1.7 and has 12 duty cycle modulated regions.FIG. 16 shows the relevant parameters of the system, the diffractionstructure, and the angle of incidence. FIG. 17 shows the duty cycle ofthe columnar structure in each modulation region. Such design outputs adiffraction order of (1,1). FIG. 13A shows the comparison of themodulated duty cycle and non-modulated duty cycle, wherein thediffraction efficiency of the modulated duty cycle of thetwo-dimensional columnar diffraction structure significantly increasesas the duty cycle thereof increases so as to show the sensitivity of thediffraction efficiency of the diffraction structure to the duty cycle.As shown in FIG. 13A, under the condition of the same total light flux,the output light flux of the non-modulated two-dimensional columnardiffraction structure with constant duty cycle will gradually decreaseduring the propagation and distribution of the waveguide, such that thebrightness of the output image is uneven within the range of the exitpupil so as to affect the user's visual experience. Accordingly, thetwo-dimensional columnar diffraction structure of the duty cyclemodulation has a constant output light flux in each modulation region,such that the brightness image is uniform for the user to view so as tosignificantly improve the user's visual experience.

One skilled in the art will understand that the embodiment of thepresent invention as shown in the drawings and described above isexemplary only and not intended to be limiting.

It will thus be seen that the objects of the present invention have beenfully and effectively accomplished. The embodiments have been shown anddescribed for the purposes of illustrating the functional and structuralprinciples of the present invention and is subject to change withoutdeparture from such principles. Therefore, this invention includes allmodifications encompassed within the spirit and scope of the followingclaims.

What is claimed is:
 1. An optical waveguide for an augmented realitydevice which comprises an optical projector for projecting a light beam,comprising: an input diffractive optical element arranged for beingaligned with said optical projector for diffracting the light beam; awaveguide substrate arranged for reflecting the light beam diffracted bysaid input diffractive optical element by means of total internalreflection; and an output diffractive optical element coupled at saidwaveguide substrate for partially diffracting the light beam as adiffracted light and partially transmitting the light beam as atransmitted light during the total internal reflection of the light beamwithin said waveguide substrate, wherein the diffracted light isdiffracted by said output diffractive optical element and is projectedout of said waveguide substrate toward a user eye, wherein thetransmitted light is continuously transmitted and reflected within saidwaveguide substrate by the total internal reflection until thetransmitted light is totally diffracted out of said waveguide substrate,so as to complete an exit pupil expansion.
 2. The optical waveguide, asrecited in claim 1, wherein said output diffractive optical elementcomprises a diffraction grating structure coupled at said waveguidesubstrate, wherein a diffraction efficiency of said diffraction gratingstructure is modulated based on parameters related to tooth profilethereof that at least one of said parameters is modulated to change withrespect to positions on said diffraction grating structure so as toobtain varied diffraction efficiency of said output diffractive opticalelement at various positions.
 3. The optical waveguide, as recited inclaim 1, wherein said waveguide substrate has a diffractive side and anopposed projecting side, wherein said input diffractive optical elementand said output diffractive optical element are located at saiddiffractive side of said waveguide substrate for guiding the diffractedlights being projected out said waveguide substrate through saidprojecting side thereof.
 4. The optical waveguide, as recited in claim2, wherein said waveguide substrate has a diffractive side and anopposed projecting side, wherein said input diffractive optical elementand said output diffractive optical element are located at saiddiffractive side of said waveguide substrate for guiding the diffractedlights being projected out said waveguide substrate through saidprojecting side thereof.
 5. The optical waveguide, as recited in claim1, wherein said output diffractive optical element has an effective areaarranged to maintain a constant light flux output per unit area thereinfor providing a uniform brightness image when the diffracted lightsbeing projected out said waveguide substrate.
 6. The optical waveguide,as recited in claim 2, wherein said output diffractive optical elementhas an effective area arranged to maintain a constant light flux outputper unit area therein for providing a uniform brightness image when thediffracted lights being projected out said waveguide substrate.
 7. Theoptical waveguide, as recited in claim 3, wherein said outputdiffractive optical element has an effective area arranged to maintain aconstant light flux output per unit area therein for providing a uniformbrightness image when the diffracted lights being projected out saidwaveguide substrate.
 8. The optical waveguide, as recited in claim 4,wherein said output diffractive optical element has an effective areaarranged to maintain a constant light flux output per unit area thereinfor providing a uniform brightness image when the diffracted lightsbeing projected out said waveguide substrate.
 9. The optical waveguide,as recited in claim 1, wherein said exit pupil expansion has asequencing energy output generated by said output diffractive opticalelement, wherein an output diffraction efficiency of said outputdiffractive optical element is optimized to:$\eta_{N} = \frac{1}{N_{T} - \left( {N - 1} \right)}$ wherein N is anoutput order of a regional modulation, N_(T) is a total number ofmodulation regions, and η_(N) is a coupling efficiency of the Nthmodulation region.
 10. The optical waveguide, as recited in claim 2,wherein said exit pupil expansion has a sequencing energy outputgenerated by said output diffractive optical element, wherein an outputdiffraction efficiency of said output diffractive optical element isoptimized to: $\eta_{N} = \frac{1}{N_{T} - \left( {N - 1} \right)}$wherein N is an output order of a regional modulation, N_(T) is a totalnumber of modulation regions, and ηN is a coupling efficiency of the Nthmodulation region.
 11. The optical waveguide, as recited in claim 4,wherein said exit pupil expansion has a sequencing energy outputgenerated by said output diffractive optical element, wherein an outputdiffraction efficiency of said output diffractive optical element isoptimized to: $\eta_{N} = \frac{1}{N_{T} - \left( {N - 1} \right)}$wherein N is an output order of a regional modulation, N_(T) is a totalnumber of modulation regions, and η_(N) is a coupling efficiency of theNth modulation region.
 12. The optical waveguide, as recited in claim 6,wherein said exit pupil expansion has a sequencing energy outputgenerated by said output diffractive optical element, wherein an outputdiffraction efficiency of said output diffractive optical element isoptimized to: $\eta_{N} = \frac{1}{N_{T} - \left( {N - 1} \right)}$wherein N is an output order of a regional modulation, N_(T) is a totalnumber of modulation regions, and η_(N) is a coupling efficiency of theNth modulation region.
 13. The optical waveguide, as recited in claim 8,wherein said exit pupil expansion has a sequencing energy outputgenerated by said output diffractive optical element, wherein an outputdiffraction efficiency of said output diffractive optical element isoptimized to: $\eta_{N} = \frac{1}{N_{T} - \left( {N - 1} \right)}$wherein N is an output order of a regional modulation, N_(T) is a totalnumber of modulation regions, and ηN is a coupling efficiency of the Nthmodulation region.
 14. The optical waveguide, as recited in claim 1,further comprising a transmission diffractive optical element thatdeflects a direction of the light beam from said input diffractiveoptical element to said output diffractive optical element.
 15. Theoptical waveguide, as recited in claim 2, further comprising atransmission diffractive optical element that deflects a direction ofthe light beam from said input diffractive optical element to saidoutput diffractive optical element.
 16. The optical waveguide, asrecited in claim 4, further comprising a transmission diffractiveoptical element that deflects a direction of the light beam from saidinput diffractive optical element to said output diffractive opticalelement.
 17. The optical waveguide, as recited in claim 6, furthercomprising a transmission diffractive optical element that deflects adirection of the light beam from said input diffractive optical elementto said output diffractive optical element.
 18. The optical waveguide,as recited in claim 8, further comprising a transmission diffractiveoptical element that deflects a direction of the light beam from saidinput diffractive optical element to said output diffractive opticalelement.
 19. The optical waveguide, as recited in claim 9, furthercomprising a transmission diffractive optical element that deflects adirection of the light beam from said input diffractive optical elementto said output diffractive optical element.
 20. The optical waveguide,as recited in claim 10, further comprising a transmission diffractiveoptical element that deflects a direction of the light beam from saidinput diffractive optical element to said output diffractive opticalelement.